I am by no means an expert on the military side of things but it is a very good starting point for anyone wanting to enter into the industry as a pilot. You mention they want to be either a pilot or navigator? If this is the case, then the ATC are definitely the way forward (also looked upon very favourably by airlines and flight training organisations).
As for the PPL, it is by no means a pre-requisite but it will give a very good idea of what flying is actually like and what kind of study is required. I would strongly suggest that he/she (you did not sepcify!) apply for every bursury/ sponsorship they can! Gliding is also a very good way to start flying and considerably cheaper - also teaches a pilot how to use their damn rudder properly from the outset!! Try the Guild of Air Pilots and Navigators for starters. I think this year's bursaries have already happened but they do offer some fantastic opportunities. The Royal Aeronautical Society are a wealth of information as are BALPA (British Airline Pilots Association). If none of the bursaries are available, then I would consier forking out for the PPL. It is very expensive (as I am sure your research has shown) but if that is what he/she wants to do for sure, then it might be a decent investment. At 16 though, there is no huge rush. University is a great opportunity for young people to join the University Air Squadron and get free flying time (up to 20 hrs a year I believe) and a degree can be an invaluable Plan B if an issue arises later that prohibits a career in the air.
I hope some of that was useful.